1. Technical Field
The present invention relates to a technique that may be used in apparatuses for detecting pattern defects in photomasks used in photolithography processes. In particular, the present invention relates to a technique for reducing the effects of ghost images that may occur when a laser is used as a light source.
2. Background Art
As an inspecting apparatus used in a semiconductor manufacturing process, an apparatus for inspecting pattern defects in a photomask (reticle) may be used. This apparatus includes an illuminating optical system for illuminating a photomask and includes a sensor for detecting an image of the photomask and outputting a signal of the image, and a pattern of the mask is thereby inspected based on the signal of the image that is output.
Recently, as patterns in semiconductor devices are made finer, the importance of inspecting a mask of a base form of a circuit pattern has been increasing. While a wavelength of illuminating light must be shortened in order to detect microdefects, a laser light source such as an ultraviolet laser may be used in view of the size of the apparatus and efficiency. Since laser light is highly coherent, when the laser light is used as a light source in a defect inspecting apparatus, interference occurs, and ghost images (multiple images generated at a place other than at the proper location) may be generated in a detected image output from a sensor. In addition, when a laser light source is used, the distribution of luminance of the laser light on an illuminated surface (or image intensity) may be uneven, which is undesirable.
The above-described ghost image does not have negative effects in the case of a binary mask, in which light and dark can be clearly distinguished, but does have negative effects in a case of a phase-shift mask (also called a “halftone mask”). Specifically, the ghost image will be a hindrance in the inspection of pattern defects of a mask.
For example, in an attenuated phase-shift mask, a masked area (an area that is not open) has a transmittance of several % to several tens of % with respect to a transmission area (an open area), and the masked area rotates the phase of the exposure light by 180 degrees. According to this function, edges of a pattern are clearly formed, whereby an even finer circuit pattern can be formed. An attenuated phase-shift mask is disclosed in Japanese Patent Application Laid-Open No. 4-136854 and Japanese Patent Application Laid-Open No. 7-140635, for example. The masked area of the attenuated phase-shift mask may transmit some amount of light, and it is thereby called a “halftone area”.
Since laser light is highly coherent, when the laser light is used as an illuminating light for a phase-shift mask, light that is transmitted through an open area of the mask and light that is partially transmitted through a halftone area interfere with each other. As a result, interference fringes are generated and are found to be the above-described ghost images. Even when the ghost image is not generated, fine fringes overlap each other, thereby generating an uneven intensity distribution (an uneven luminance distribution) of light transmitted through the photomask.
A technique for reducing the generation of the ghost images is proposed in Japanese Patent Application Laid-Open No. 11-72905, for example. In this technique, laser light is emitted from a light source and is passed through an integrator (a fly-eye lens) and a rotating phase plate so that the phase of the laser light is randomly changed, whereby temporal coherence is decreased. In Japanese Patent Application Laid-Open No. 11-72905, as a technique for decreasing effects of an uneven luminance distribution, a technique for evening out deviation of the luminance distribution by using an oscillating mirror is disclosed.